Metrology targets are used to verify the accuracy of production steps and are designed to be optically measureable. Common types of metrology targets provide optical contrast between target elements and their background regions.
The inventors have found that the common practice of separating target elements by empty background regions results in different kinds of metrology errors. The inventors have further discovered that these metrology errors may be at least partly overcome by different and advantageous target designs and design principles which are presented below. It is noted that while the disclosed ideas are mainly exemplified for segmented target elements which are separated by empty background regions, they are likewise implementable to any large featureless region such as originally unsegmented target elements, and regions in different layers of the wafer. As the terms “empty” and “segmented” depend on the respective scale, it is further noted that the dimensions which are mentioned below may be adjusted according to specific characteristics of layer and target design and of production processes, and adapted to any design and production technologies.
FIGS. 1A and 1B illustrate a dishing effect and a rotation term effect, respectively, as resulting from the prior art practice of leaving empty background regions between target elements. FIG. 1A schematically illustrates prior art target 90, comprising segmented target elements 90A separated by empty background regions 90B. Typical and non-limiting dimensions for the illustrated target parts are on the scale of micrometers, e.g., the widths of target element 904 and of empty background regions 90B may be within a range of 0.5-1 μm (in their shorter dimension) and the respective target pitch may be within a range of 1-2 μm. The graphical representations shown in FIG. 1A are of a “dishing effect” that results from empty background regions 90B, namely uneven or non-planar layer regions 85 which form above empty background regions 90B due to characteristics of process steps such as etching, deposition or polishing. In the illustrated example, simulation results for deposited layers 83 are presented (approximate scales of layer height h versus displacement y are given), showing significant accuracy issues, depending on the dishing depth (top right—no dishing, middle right—5 nm dishing, bottom right—15 nm dishing). The overlay (OVL) values represent only this OVL inaccuracy, since the induced OVL value in the simulation was zero. The deeper the dishing, the more significant is the measured inaccuracy OVL.
Another effect, illustrated in FIG. 1B, results in a large scale, global effect of such uneven layer regions 85. For example, during the Chemical Mechanical Polishing/Planarization (CMP) process, empty regions 90B between target features 904 may become somewhat concave and introduce asymmetric polishing of target elements 904 which border these empty regions 90B. The asymmetric polishing results in measurement errors that are at least partly quantified by a “rotation term” of inaccuracy, which reflects a cumulative effect of asymmetric polishing of target elements 90A bordering empty regions 90B, relating to the rotational movement of the polish pad. FIG. 1B illustrates experimental results that illustrate the rotation term that originates from asymmetric polishing of empty regions in target 90. In the illustrated case, targets 90 are Box-in-Box targets (left of FIG. 1B) which have a central empty region (as well as peripheral empty regions) in which the dishing effect occurs. Image 95 is an overview of a wafer, in which the overlays relating to individual targets 90 are marked by arrows 87. The rotation term is evident in the overall circular pattern of overlays 87, having larger overlays 87 with increasing distance of targets 90 from the center of the wafer. The rotation term originates from asymmetric polishing of empty regions 90B in targets 90. In the example illustrated in FIG. 19, the rotation term (measured as a ratio between the rotational overlay and the distance from the center of the wafer) is ca. 0.02 ppm.
Commonly used imaging and scatterometry metrology targets such as BiB, AIM, AlMid, SCOL or DBO may have relatively large empty regions 90B without any pattern which provide contrast for optical measurements. These empty regions are typically 4-20 μm long and 300 nm-2 μm wide. Such dimensions commonly cause the dishing effect and resulting rotation term illustrated above.
Thus, there is a long felt need for a target design with improved robustness for the CMP (Chemical Mechanical Polish) process.